Learning Objectives:
Explain ultrasound physics relevant to EUS: piezoelectric effect, frequency, resolution, attenuation, echogenicity, doppler - color and spectral, aliasing
Become proficient in “knobology”. Specifically obtain optimal images by selecting the proper probe, adjusting image mode, gain, time gain compensation, and focus.
Recognize common ultrasound artifacts including: reverberation, side lobe, mirror, shadowing, enhancement, ring down
Basic Physics principles:
Probe emits ultrasound signal, detects depth based on time to return after being reflected by object
Amount that reflects back depends on material and angle of the beam relative to the object (angled object will scatter reflection and not all the sound waves will make it back to the probe
Ultrasound transducers have specific frequencies
Frequency: the number of sound waves per second
Wavelength: the length or distance of a single wave
Frequency = speed of sound/wavelength
Short wavelength = high frequency
Long wavelength = low frequency
Piezoelectric effect: in ultrasound, the piezoelectric effect allows the transducer to convert electrical energy into sound waves that travel into the body and then convert returning echoes back into electrical signals, which are used to create real-time images of internal organs, tissues, or blood flow.
Frequency: frequency refers to the number of sound wave cycles per second measured in megahertz. Affects image resolution and depth of penetration. Higher frequencies provide clearer images but do not penetrate as deeply, while lower frequencies penetrate deeper into the body but produce lower-resolution images
Resolution: refers to the ability to distinguish between two closely spaced structures - high resolution allows for more detailed and accurate images. Resolution improves with higher frequencies, helping to differentiate structures that are close together along the path of the ultrasound beam.
Attenuation: refers to the fact that sound waves get weaker as they travel through the body - each medium in the body has a certain attenuation, meaning a different speed at which it decreases ultrasound intensity
Bone and air have HIGH attenuation, meaning they quickly decrease the intensity of ultrasound
Blood has low attenuation, fat and liver have slightly higher attenuation
Impedance: how much a tissue resists the sound wave passing through it. When there’s a big difference in impedance between two tissues—like soft tissue and bone—more sound is reflected back, which helps create the image; for example, the bright white line seen when scanning over bone is due to its high impedance.
Echogenicity: how bright or dark a structure appears on the image, based on how much sound it reflects. (pictures)
For example, fluid-filled structures like cysts appear black (anechoic) because they don’t reflect sound, while solid structures like bone reflect more sound and appear brighter (hyperechoic).
Doppler - color and spectral: (pictures)
Color Doppler shows blood flow direction and speed as different colors overlaid on the image—red usually means flow toward the transducer and blue means flow away—helping to quickly identify areas of normal or abnormal blood flow.
In color doppler, RED = TOWARDS transducer and BLUE = AWAY from transducer. These do not necessarily correlate with venous and arterial flow!
Spectral Doppler provides a graph of blood flow velocity over time, allowing precise measurement of flow speed and patterns, which is useful for assessing things like valve function or vessel narrowing. Includes pulse wave and continuous wave doppler.
Aliasing: inherent to Doppler - when blood is flowing too fast for the Doppler to measure correctly due to insufficient intermittent sampling rate. This makes the colors or waveforms look wrong—for example, a flow that should be all red might suddenly show blue, making it harder to interpret accurately.
Knobology and image acquisition:
Knobology: understanding and using the controls on an ultrasound machine to get the best possible image. Key controls include gain, which adjusts the brightness of the image; depth, which sets how deep the sound waves go; and focus, which sharpens the image at a certain level. Other important settings include time gain compensation (TGC) to adjust brightness at different depths and freeze to capture still images. Knowing how to adjust these settings helps improve image quality and diagnostic accuracy.
Image Mode (pictures)
B-mode (Brightness mode): the most common mode and default on the machines - this is what creates 2D grayscale images of structures based on the strength of returning echoes
M-mode (Motion mode): shows the motion of structures over time along a single line. We use this in cardiac imaging to measure EPSS and TAPSE
Color Doppler, Spectral Doppler, Power Doppler
Gain: controls the brightness of the image by adjusting the amplification of the returning echoes. Increasing the gain makes the entire image appear brighter, while decreasing it makes it darker. Proper gain settings are important—too much gain can make the image look washed out, and too little can make structures hard to see.
Time gain compensation: allows you to adjust the brightness of the image at different depths. As sound waves lose strength the deeper they go, TGC helps make deeper structures appear just as bright as shallow ones. This creates a more uniform image, so you can clearly see structures at all levels.
Focus: refers to the area where the ultrasound beam is the narrowest and image resolution is the highest. Adjusting the focus to the level of the structure you're examining improves clarity and detail, making it easier to identify small or subtle findings. Proper focus placement is key for accurate imaging. This is a small sideways arrow on our machines on the right side where the depth measurement is seen.
Transducer types (clip art for probes)
Linear probe: high frequency, for superficial structures like blood vessels, soft tissue, pleural line, and MSK exams. Good detail, limited depth.
Curvilinear probe: lower frequency, for abdominal, pelvic, and deep organ imaging. Good penetration, moderate resolution.
Phased array probe: low frequency, small footprint, mainly for cardiac imaging because it fits between the ribs
artifacts (Need examples)
Reverberation: appears as multiple, equally spaced echoes caused by sound bouncing back and forth between two strong reflectors, like the pleura or a needle.
Side lobe: occurs when off-axis sound beams reflect off a strong structure and are mistakenly placed in the main beam path, creating false echoes. (ocular example)
Mirror: creates a duplicated image of a structure on the opposite side of a strong reflector, like the diaphragm, making it look like a second liver.
Shadowing: appears as a dark area behind a highly attenuating structure, such as bone or a gallstone, where the sound waves are blocked.
Enhancement: seen as increased brightness behind a fluid-filled structure like a cyst or bladder, because the sound waves pass through fluid with minimal loss.
Edge artifact: appears as a shadow-like line extending from the edges of a curved structure, such as a gallbladder or a vessel. It occurs when sound waves bend (refract) at the edges, causing a loss of signal directly behind them, and can be mistaken for pathology if not recognized.
Ring down: shows as a continuous, bright vertical echo extending deep to a gas bubble, caused by resonance of trapped air.
B lines in thoracic ultrasound are caused by this artifact and phenomenon.
Gas in the bowel on abdominal ultrasound can also lead to bright lines extending deep, obscuring structures behind them
The bright line from air when scanning necrotizing fasciitis is also an example of this artifact
Resources:
ACEP Sonoguide: Ultrasound Physics and Techincal Facts for the Beginner
Core Ultrasound: Basics
POCUS 101: Basic Principles of Ultrasound Physics and Artifacts Made Easy
POCUS 101: Ultrasound Knobology, Probes, and Modes Made Easy
The Ohio State University Ultrasound Interest Group: The Basics of Ultrasound
Malin and Dawson iBook Volume 1, Chapter 8: Physics
US Physics and Knobology Lecture by Geoff Hayden
Author: Ryan Abbott, DO (Class of 2027)
Peer Editing by: Christine Jung, MD-FDP